Continuous processing of vesiculated beads

Gous, Karen (2003-04)

Thesis (MScIng)--University of Stellenbosch, 2003.

Thesis

ENGLISH ABSTRACT: Titanium dioxide (TiO2) is extensively used as pigment in paint formulations, but due to the high cost associated with it, along with its’ depleting natural resources, paint manufacturers are seeking substitutes that can at least partially replace this pigment. Vesiculated beads have been successfully used in the past as a replacement for the TiO2-pigment. These beads are spherical polymer particles that contain a multitude of aerated micro voids within the polymer shell. The aerated voids cause effective scattering of light inside the particles, presenting the beads with a white appearance. It was found that low levels of TiO2 could be encapsulated in the particles as a means of increasing the whiteness and hiding power of the beads in the wet- and dry state. Because the beads are about two-thirds air by volume and contain only small amounts of TiO2, it also presents a method of reducing the formula cost. The beads are produced with an emulsification process where by an organic phase containing unsaturated polyester resin and styrene monomer is emulsified within an aqueous phase. This phase consists mainly of de-ionised water and stabilisers that assist in forming a stable emulsion of organic droplets in the continuous aqueous phase. A polyamine is also added to the system to achieve the uptake of water droplets inside the organic phase. It is this water that is replaced with air when it evaporates from the beads upon drying, and leaves the particles with air-filled vesicles. Emulsification is currently achieved locally with the use of a Cowles disperser system or an emulsion reactor set-up with the application of a specified stirring speed for a specific period of time. These stirring specifications are manipulated so that the organic globules are subjected to a specific shear rate and consequently are broken down to the required particle size. The polymeric reaction is initiated with the addition of a free-radical initiator and redox activator and the product is left overnight to allow completion of the reaction and subsequent formation of slurry containing solid polymer particles. In this study, homogenisation was investigated as a means of emulsifying and producing vesiculated beads in a continuous process. Homogenisation is defined as the act of breaking down globules into smaller particles under pressure and produces a product with evenly dispersed (homogeneous) fine particles. This process could therefore also be used to produce particles with a smaller average particle size than those obtained by the existing Cowles- and emulsion reactor manufacturing processes. These batch processes produce vesiculated beads with an average particle size between 3 and 10 micron on industrial scale. With the incorporation of the homogeniser in a continuous process it would be possible, not only to produce particles in the order of 1 micron required for the addition to gloss paint, but it would also have the added advantages of easy cleaning, higher production rates and the use of smaller equipment. The most important operating parameters when using a homogenisation system were determined using a laboratory scale homogeniser set-up. These parameters included the geometry of the internals of the homogeniser, the number of passes and the flow rate. From the information and results obtained from the batch experiments a semicontinuous bench-scale homogeniser set-up was designed. This experimental set-up consisted of a homogeniser, high-pressure pump, continuous feed section for the initiator and a loop-reactor equipped with a heating mantle to facilitate continuous curing of the product. Vesiculated beads with properties similar to those obtained by the existing batch processes, but with an average particle size in the order of 1 micron, were produced successfully with this semi-continuous set-up. Although the beads were not entirely cured after leaving the loop-reactor it is believed that with increased heating and an increase in the length of the reactor, this problem can be addressed. The results obtained with the semi-continuous process could be used in future in the design and construction of a continuous pilot plant for the production of vesiculated beads.

AFRIKAANSE OPSOMMING: Titaandioksied (TiO2) word in groot hoeveelhede as pigment in verf gebruik, maar as gevolg van die hoë koste van hierdie pigment en die uitputting van die natuurlike bronne, is verfvervaardigers op soek na alternatiewe. Polimeerpartikels met lugholtes daarin vasgevang, is in die verlede suksesvol as plaasvervanger vir die pigment gebruik. Hierdie partikels is sferiese polimeerpartikels met ‘n menigte klein lugholtes wat effektiewe verstrooiing van lig binne die partikels veroorsaak. Dit verskaf aan die partikels ‘n wit voorkoms en daar is gevind dat klein hoeveelhede TiO2 binne die partikels vasgevang kan word om die witheid in die nat- en droë fase te verhoog. Omdat die partikels uit ongeveer twee derdes lug bestaan en slegs klein hoeveelhede TiO2 bevat, word ‘n vermindering in produksiekoste verkry as dit in verf gebruik word. Die polimeerpartikels word geproduseer met ‘n emulsifiseringsproses waarby ‘n organiese fase, bestaande uit onversadigde poliëster en stireen monomeer, in ‘n waterfase geëmulsifiseer word. Laasgenoemde bestaan hoofsaaklik uit gedeïoniseerde water en stabiliseerders wat die vorming van ‘n stabiele emulsie van organiese druppels in die kontinue waterfase bewerkstellig. ‘n Poli-amien word ook by die sisteem gevoeg om die organiese fase in staat te stel om water op te neem. As hierdie water dan van die partikels verdamp wanneer dit droog word, word dit deur lug verplaas en laat dit die partikels met ‘n menigte lugholtes agter. In Suid-Afrika, word emulsifisering tans bewerkstellig deur die gebruik van ‘n “Cowles” menger of ‘n emulsiereaktorsisteem waar ‘n spesifieke roerspoed vir ‘n vasgestelde tydperk aangewend word. Hierdie roerspesifikasies word so gekies dat ‘n bepaalde skuifkrag op die organiese druppels uitgeoefen word en dit dus tot die verlangde partikelgrootte opgebreek word. As die bepaalde partikelgrootte bereik is, word die reaksie geïnisieer deur die byvoeging van ‘n vry-radikaal inisieerder en ‘n redoksaktiveerder. Die produk word dan oornag gelaat sodat die reaksie voltooi kan word en die soliede polimeerpartikels binne die kontinue waterfase gevorm kan word. Tydens hierdie studie is homogenisering ondersoek as ‘n moontlike metode om emulisifisering te bewerkstellig en sodoende die polimeerpartikels te produseer. Homogenisering word gedefinieer as ‘n proses waartydens partikels afgebreek word onder hoë druk en gevolglik lei dit tot die vorming van ‘n produk bestaande uit klein partikels uniform versprei deur die produk. Die moontlikheid bestaan dus dat homogenisering in ‘n proses gebruik kan word om kleiner partikels te produseer as wat moontlik is met die bestaande Cowles- of emulsiereaktor prosesse. Hierdie enkelladingsprosesse word gebruik om polimeerpartikels met ‘n gemiddelde partikelgrootte tussen 3 en 10 mikron op industriële skaal te produseer. Deur die homogeniseerder in ‘n kontinue proses te gebruik sal so ‘n proses gebruik kan word nie net om partikels met ‘n grootte van ongeveer 1 mikron (noodsaaklik vir glansverf) te produseer nie, maar hou dit ook verdere voordele in soos hoër produksie, kleiner toerusting en die gemak waarmee so ‘n sisteem skoongemaak kan word. ‘n Laboratorium-skaal enkelladings homogeniseerder is gebruik om die belangrikste bedryfsparameters van ‘n homogeniseringsisteem te bepaal. Hierdie parameters sluit in die geometrie van die interne dele van die homogeniseerder, asook die vloeitempo en die aantal deurgange deur die homogeniseeder. Die resultate van die enkelladings eksperimente is gebruik om ‘n semi-kontinue loodsaanleg skaal opstelling met ‘n hoë druk pomp, homogeniseerder, kontinue toevoer seksie vir die inisieërder en ‘n kontinue buisreaktor om die volledige verloop van die reaksie te verseker, te ontwikkel. Polimeerpartikels gevul met lugholtes is geproduseer met bogenoemde proses en produkeienskappe vergelykbaar met die van die bestaande enkelladingsprosesse is verkry. Die enigste verskil tussen die produkeienskappe van die verskillende prosesses was die gemiddelde partikelgrootte met die partikels geproduseer met die homogeniseerderproses in die order van 1 mikron en die van die ander prosesse veel groter. Die resultate verkry met die semi-kontinueproses sal in die toekoms gebruik word vir die ontwerp en bou van ‘n ten volle kontinue loodsaanleg.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/16332
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